Electrophotographic marking is a well-known and commonly used method of copying or printing documents. Electrophotographic marking is performed by exposing a light image representation of a desired document onto a substantially uniformly charged photoreceptive member. In response to exposure by the light image representation, the photoreceptive member discharges so as to create an electrostatic latent image of the desired document on the surface of the photoreceptive member. The development material, having toner, is then deposited onto the electrostatic latent image so as to form a toner image. The toner image is then transferred from the photoreceptor onto a substrate such as a sheet of paper. The transferred toner image is then fused to the substrate to form the completed document. Fusing of the toner image to the substrate is typically accomplished by a combination of heat and/or pressure. The surface of the photoreceptive member is then cleaned of residual developing material and recharged in preparation for production of another document.
Electrophotographic printing machines can employ discharge area development to develop the exposed latent image. In discharge area development (DAD) the charged photoreceptive member is exposed where the marking material is to appear on the final document. The marking material of toner is developed into the discharged area to form the image for transfer to a substrate. The discharge of the photoreceptive member occurs over time whereby the exposed portions of the photoreceptive member have a voltage varying over time to define a discharge curve. The print quality of an electrophotographic printing machine employing DAD is dependent on the voltage of the exposed portions of the photoreceptive member extending sufficiently far into the tail of the discharge curve. In other words, the image area must be sufficiently discharged for sufficient development and thereby acceptable image quality. This is achieved by the extension of the voltage of the exposed portions of the photoreceptive member sufficiently far into the discharge curve thereby providing adequate contrast potential in the latent image. In addition, extension of the voltage at the exposed portions sufficiently far into the discharge curve predicts the surface voltage of the photoreceptive member at the development station within preselected voltages. These preselected voltages are preferred for acceptable print quality. The sufficient extension of the voltage at the exposed portions into the tail of the discharge curve requires adequate time to be provided for the photo-generated carriers in the photoreceptive member to be injected into and then transit the transport layer of the photoreceptive member. Typically however, in digital imaging systems whereby exposure is accomplished by a laser, charges amounting to a CV's worth are generated in the photoreceptor as a result of the substantial quantity of charge generated by exposure with a laser.
Electrophotographic printing machines are therefore designed to provide sufficient time for the majority of the photo-generated carriers to transit the transport layer of the photoreceptive member prior to the latent image arriving at the development station. The provision of sufficient time for transit of the photo-generated carriers through the transport layer allows for maximum contrast of voltage potentials in the exposed and unexposed portions of the latent image. In addition, the provision of sufficient time for charge transit results in stable print quality as the potential on the surface of the photoreceptive member does not rapidly change in time. However, particularly with the color systems, having narrow tolerances required for high quality color hardcopies using digital exposure systems, the effect of charge transit time limitations can impose architectural limitations on the electrophotographic printing machine.